1. Field of the Invention
The present invention relates to a fuel cell system with a device for supplying fuel to the fuel cell.
2. Description of the Related Art
Fuel cells have long been known and have gained considerably in significance in recent years, especially in the area of the automobile industry.
In a fuel cell, current is produced by a chemical reaction. This involves converting a fuel and an oxidizing agent into electrical energy and water as reaction products. A fuel cell essentially comprises an anode part, a membrane and a cathode part. The membrane consists of a gas-impermeable and proton-conducting material and is arranged between the anode and the cathode in order to exchange ions. On the anode side, the fuel is fed in, while on the cathode side the oxidizing agent is fed in. At the anode, protons or hydrogen ions are generated by catalytic reactions and move through the membrane to the cathode. At the cathode, the hydrogen ions react with the oxygen and water is formed. The electrons given off during the reaction can be conducted as electric current through a load, for example the electric motor of an automobile.
Used for example as gaseous reactants for the fuel cell are hydrogen as the fuel and oxygen or air as the oxidizing agent.
If it is desired to operate the fuel cell with an energy source which is easily available or easily stored, such as natural gas, methanol or the like, these hydrocarbons must generally first be converted, initially into a hydrogen-rich gas, in a corresponding device for producing/preparing a fuel for a fuel cell, as is known for example from DE 198 23 499 A1. However, fuel cells which do not require such a prior conversion also already exist. For instance, EP 0 778 631 A1 discloses a fuel cell which is operated with a methanol/water mixture as the fuel.
The fuel or the starting material for the fuel is usually not introduced into the device for producing/preparing the fuel or the fuel cell in its pure form but as a motor-fuel/water mixture. For this reason, the motor fuel or the starting material for the fuel must initially be mixed with a suitable amount of water.
For the delivery of liquid media, so-called dual pumps, which respectively comprise two separate pumps within a common housing and are usually operated by means of a common drive, have been used for many years. For instance, DE 298 18 704 U1 discloses a dual pump which is used for example for the delivery of two different cooling flows in a cooling system. DE 197 10 675 A1 discloses a dual pump which is intended in particular for the delivery of heating water and comprises two pumps which are designed for delivery flows of distinctly different magnitudes and the rotational speeds of which are regulated individually or jointly, according to choice, by a pump control system and are operated over a great range of the total delivery rate. Furthermore, DE 195 36 997 C1 discloses a dual pump of an axial-piston type of design, which provides a joint suction line for both pumps, in which line a centrifugal pump is arranged for creating a suitable admission pressure for the two pumps of the dual pump. Furthermore, DE 30 41 832 A1 discloses an axial-piston dual pump for high-performance hydraulics. Finally, DE 35 44 016 C2 is concerned with a pump arrangement designed as a dual pump for the metered dispensing of at least 2 components with an adjustable mixing ratio. For this purpose, the two pumps of the dual pump are respectively provided with a swept-volume adjusting device, the two adjusting devices being connected to each other for joint symmetrical adjustment.
The present invention is based on the object of improving a fuel cell system of the generic type to the extent that a required motor-fuel/water mixture can be prepared and introduced into the device for producing/preparing the fuel or directly into the fuel cell in as simple and low-cost a way as possible.
This object is achieved according to the invention by a fuel cell system with a fuel cell which has an anode part with a supply line and a discharge line for a fuel and a cathode part with a supply line and a discharge line for an oxidizing agent, and with a device for supplying the fuel to the fuel cell either directly or via a device for producing/preparing the fuel, which is connected on the outlet side to the fuel supply line and on the inlet side to the device for supplying fuel, wherein the device for supplying fuel has a motor-fuel tank from which a motor-fuel supply line is led out. Also provided is a dual pump, which is likewise arranged in the motor-fuel supply line and is also connected to a water supply line. Finally, an intermediate tank is provided for the intermediate storage of the motor-fuel/water mixture and is likewise arranged in the motor-fuel line, downstream of the dual pump in the direction of flow.
The device according to the invention makes it possible in a simple and low-cost way to prepare a required motor-fuel/water mixture and to make it available to the device for producing/preparing the fuel or directly to the fuel cell.
In this case, the starting material for the fuel of the fuel cell is initially stored as motor fuel in the motor-fuel tank. If hydrogen is used as fuel for the fuel cell, the motor fuel may be, for example, methanol, methane, gasoline, natural gas, coal gas, biogas or the like.
The motor fuel is fed into the dual pump via the motor-fuel supply line. The dual pump is also connected to a corresponding water supply line. Such a dual pump has the effect that the motor fuel pumped out of the motor-fuel tank, serving as the starting material for the fuel, is mixed with the water, initially to form a motor-fuel/water mixture, before entering the device for producing/preparing the fuel or into the fuel cell.
This mixture produced in the dual pump is then intermediately stored in an intermediate tank, before it is fed into the device for producing/preparing the fuel or into the fuel cell.
The intermediate tank serves as a buffer store, so that a sufficient amount of motor-fuel/water mixture is always available for operating the device for producing/preparing the fuel or for operating the fuel cell. The presence of a sufficient amount of motor-fuel/water mixture is of significance in particular in the starting phase or running-up phase of the fuel cell system. During the starting phase of the fuel cell systemxe2x80x94especially during cold startingxe2x80x94it is necessary to introduce the motor-fuel/water mixture as quickly as possible into the device for producing/preparing the fuel or into the fuel cell, in order that the fuel cell reaches its optimum performance capability as quickly as possible. The use of an intermediate tank in the motor-fuel supply line achieves the effect that a sufficient amount of motor-fuel/water mixture always remains in the fuel cell systemxe2x80x94especially also after switching off the fuel cellxe2x80x94, so that when the fuel cell system is started again the motor-fuel/water mixture does not first have to be produced but is available right away.
Water which is discharged from the fuel cell during the fuel cell process and is recovered from the exhaust-gas flows from the fuel cell is advantageously used for the dual pump. This solution is explained in more detail in the further course of the description, without intending to restrict the invention to this one solution. The water supplied to the dual pump via the water supply line may, however, also be supplied from an external water source.
The device according to the invention for supplying fuel makes it possible in a simple way to prepare a motor fuel before it is further used as an intermediate product by mixing in process water right at the beginning of the process sequence in order to produce the corresponding motor-fuel/water mixture.
A further pump may advantageously be provided in the motor-fuel supply line upstream of the dual pump in the direction of flow of the motor fuel. By means of this pump, which is designed for example as a motor-fuel pump, the motor fuel or the starting material for the fuel is pumped out of the motor-fuel tank and passed into the dual pump. With a correspondingly powerful dual pump, this function can also be assumed by the dual pump.
During the starting phase of the fuel cell system, the dual pump and/or the further pump is advantageously operated by means of an electrical power source that is independent of the fuel cell, preferably a chargeable battery.
The intermediate tank may preferably be arranged in or on the motor-fuel tank. In this way, a very space-saving type of design can be accomplished, which is of advantage in particular when the device is used in a motor vehicle, since usually only a very limited installation space is available there.
In a further development, a water tank may be provided in the water supply line.
Such a water tank is of advantage for example when water which is discharged from the fuel cell during the fuel cell process and recovered from the exhaust-gas flows of the fuel cell is used for producing the motor-fuel/water mixture. This process water is passed via the water supply line into the dual pump, where it is mixed with the starting material for the fuel.
Such process water is only produced, however, during the operation of the fuel cell, with the result that in the starting phase of the fuel cell system in particular it is not yet available. For this reason, the water tank serves as a corresponding water reservoir, from which water can be taken, in particular during the starting phase of the fuel cell system, to allow the required motor-fuel/water mixture to be made available as quickly as possible.
However, it may also be appropriate to dispense with such a water tank and accomplish the feeding of water into the dual pump by appropriate dimensioning of the water supply line alone. In particular whenever the device for supplying fuel into a fuel cell is used in conditions where there are very low ambient temperatures, that is to say temperatures below freezing, there is the risk that water located in a water tank may freeze. In such a case, the frozen water would first have to be thawed, which is relatively time-consuming. If a fuel cell system provided with such a device for supplying fuel is used at very low temperatures, whenever possible a separate water tank should therefore not be used.
To allow an adequate amount of motor-fuel/water mixture nevertheless to be made available already during the starting phase of a fuel cell system in such a case, the water tank must be dimensioned such that it can take enough motor-fuel/water mixture for the fuel cell system to be operated with the stored amount until sufficient process water for further operation is produced by the fuel cell process.
If a water tank is nevertheless used in the water supply line, the problem of possible freezing of water located in the water tank can be countered, for example, by the water tank always being emptied when stationary and the water tank only being filled during operation of the fuel cell system. This means that, in particular in the time periods after switching off the fuel cell system and also during the starting phase of the fuel cell system, the water tank is empty. As a result, freezing of water in the water tank can no longer occur. In another development of the invention, it is conceivable to design the water tank in the form of an expandable vessel. Should water located in the water tank freeze and expand as a result, damage to the water tank can be prevented by its ability to expand.
The water supply line and/or the water tank is preferably connected to a drain valve. The process water can be drained off via the drain valve, for instance when the intermediate tank is full. Furthermore, to avoid freezing, it is possible for water to be drained from the water supply line and/or the water tank if the fuel cell system is switched off for a relatively long time or when there are low temperatures.
In a further development, a further pump may be provided downstream of the intermediate tank in the direction of flow. It is ensured by means of this pump that the motor-fuel/water mixture is introduced at a required pressure into the device for producing/preparing the fuel or the fuel cell. It is conceivable, for example, to use such a pump whenever the motor-fuel/water mixture only has a pressure of xe2x89xa61 bar when it leaves the intermediate tank. If, on the other hand, the pressure after the intermediate tank is, for example, already 1 bar or more, such a pump is generally not required. In this case, the pump can in fact remain switched off and only has to be activated in emergencies. For this purpose, a bypass line bypassing the pump may be provided, sending the motor-fuel/water mixture past the pump if the pressure is sufficiently high. If the pressure drops below a certain limit value, the mixture can then be passed via the pump by adjusting a corresponding valve.
In a fuel cell system according to the invention, it is possible in a simple and low-cost way to make an adequate amount of motor-fuel/water mixture always available to a device for producing/preparing the fuel.
In the device for supplying fuel, the motor-fuel/water mixture is initially prepared in the required mixing ratio. Subsequently, this motor-fuel/water mixture is introduced into the device for producing/preparing the fuel, where it is converted into a hydrogen-rich gas. Then, this hydrogen-rich gas is introduced into the fuel cell via the fuel supply line.
The device for producing/preparing the fuel usually has a number of reactor elements, which are connected to one another via corresponding lines, so that the fuel flows through the individual reactor elements while it is being produced or prepared.
Heat is required in some of the reactor elements. One of these reactor elements is, for example, an evaporator. Such an evaporator is required for instance whenever hydrogen is to be re-formed from methanol or the like. Another reactor element is a reformer, which is arranged downstream of the evaporator and in which the motor-fuel/water mixture is re-formed into a hydrogen-rich gas.
In other reactor elements, on the other hand, heat is released. These reactor elements may be designed, for example, as selective oxidation elements. This reactor element is required if the hydrogen is produced by the process of partial oxidation. In this case, the gas is initially cleaned in a shift reactor for the removal of CO by a homogeneous water-gas reaction and is finely cleaned in the following stagexe2x80x94that of selective oxidation.
A water separator, which is connected in particular to the water supply line of the dual pump, may preferably be provided in the fuel discharge line and/or the oxidizing-agent discharge line of the fuel cell. This water separator allows the process water produced during the fuel cell process to be collected and subsequently put to further use. This is because not only heat but also water is produced, inter alia, during the operation of the fuel cell, and is carried away in the form of water vapor in the exhaust-gas flow of the oxidizing agent and/or of the fuel. To transform the water vapor into the liquid state, in order that it is available for other processes, for example the mixing in of the process water at the beginning of the process sequence, after leaving the fuel cell the gas flow containing the water vapor flows through the water separator, in which the water vapor is condensed into liquid water.
This water is preferably passed via the water supply line to the dual pump and used there for producing the motor-fuel/water mixture.
A fuel cell system according to the invention, as described above, can preferably be used in a vehicle or for a vehicle, in particular as a power source for an electric vehicle. On account of the rapid development of fuel cell technology in the automobile sector, such a use offers particularly good application possibilities. Nevertheless, other application possibilities are also conceivable. To be mentioned here are, inter alia, fuel cells for mobile devices such as computers or the like through to stationary facilities such as power plants. Here, the fuel cell technology is particularly suited for the decentralized supplying of energy to houses, industrial plants or the like.
In a preferred way, the present invention is used in conjunction with fuel cells with a polymer membrane (PEM). These fuel cells have a high electrical efficiency, cause only minimal emissions, have optimum part-load characteristics and are essentially free from mechanical wear.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.